a. Field of the Invention
The present invention relates generally to anti-vandalism detectors and, in particular, to systems and methods for detecting breakage and defacing of materials such as glass and plastic.
b. Current State of the Relevant Field
The direct annual costs of vandalism run in the billions of dollars per year in the United States alone. Conventional security techniques, such as direct lighting and intruder alarms, are often used to deter vandalism. However, there are many locations such as public transit vehicles, where such conventional techniques are ineffective or impractical. Conventional techniques used to protect glass and plastic windows and public seating are summarized below.
i. Glass Anti-Vandalism Techniques
A common component of a security system is a glass-break detector. Older models relied on mechanical disturbance or vibration. Newer models are available that use digital signal processing to protect a whole room by listening for the sound of breaking glass. However, the sound frequencies generated by breaking glass are rather indistinct. As a result, even these newer sensors are vulnerable to false alarms from a variety of sources such as music and must be tuned carefully to detect all possible glass breakage events.
Glass windows can be scratched by a vandal using glass-cutting tools, available for a few dollars at many hardware stores. Often the glass is ruined. Expensive professional services are available to polish out such scratches, provided that the glass started out thick enough, has enough remaining thickness and under many circumstances is not of the automotive safety type. Much research has focused on films or coatings that can be replaced to restore a scratched pane to original condition.
ii. Plastic Anti-Vandalism (Scratch) Detection
Many applications use a variety of non-glass panes for example plastic such as Lexan.TM.. The advantage of plastic panes is shatter resistance. However, plastic panes are even more vulnerable to scratching than glass. A simple key can leave a deep mark. Scratch detection is difficult because the associated sound frequencies are even less distinct than those associated with glass scratching.
iii. Seat Slash Detection
There is no cost effective system for sensing the vandalism, such as by slashing or cutting, of public seating in buses, bus stops and elsewhere. Perhaps as a result, public seating is often made from hard, easy to clean and vandalism-resistant materials such as reinforced plastic.